Corrosion problems in petroleum refining operations associated with naphthenic acid constituents and sulfur compounds in crude oils have been recognized for many years. Such corrosion is particularly severe in atmospheric and vacuum distillation units at temperatures between 400.degree. F. and 790.degree. F. Other factors that contribute to the corrosivity of crudes containing naphthenic acids include the amount of naphthenic acid present, the concentration of sulfur compounds, the velocity and turbulence of the flow stream in the units, and the location in the unit (e.g., liquid/vapor interface).
In the distillation refining of crude oils, the crude oil is passed successively through a furnace and one or more fractionators such as an atmospheric tower and a vacuum tower. In most operations, naphthenic acid corrosion is not a problem at temperature below about 400.degree. F. Traditional nitrogen-based filming corrosion inhibitors are not effective at temperatures above 400.degree. F. and the other approaches for preventing naphthenic acid/sulfur corrosion such as neutralization present operational problems or are not effective.
It should be observed that the term "naphthenic acid" includes mono and di-basic carboxylic acids and generally constitutes about 50% by weight of the total acidic components in crude oil. Many of the naphthenic acids may be represented by the following formula: ##STR2## where R is an alkyl or cycloalkyl and n ranges generally from 2 to 10.
Many variations of this structure and molecular weight are possible. Some practitioners include alkyl organic acids within the class of naphthenic acids.
Naphthenic acids are corrosive between the range of about 400.degree. F. (210.degree. C.) to 790.degree. F. (420.degree. C.). At the higher temperatures the naphthenic acids are in the vapor phase and at the lower temperatures the corrosion rate is not serious. The corrosivity of naphthenic acids appears to be exceptionally serious in the presence of sulfide compounds, such as hydrogen sulfide, mercaptans, elemental sulfur, sulfides, disulfides, polysulfides and thiophenols.
Efforts to minimize or prevent the naphthenic acid/sulfur corrosion have included the following approaches:
(a) blending of higher naphthenic acid content oil with oil low in naphthenic acids; PA1 (b) neutralization and removal of naphthenic acids from the oil; and PA1 (c) use of corrosion inhibitors.
Because these approaches have not been entirely satisfactory, the accepted approach in the industry is to construct the distillation unit, or the portions exposed to naphthenic acid/sulfur corrosion, with the resistant metals such as high quality stainless steel or alloys containing higher amounts of chromium and molybdenum. However, in units not so constructed there is a need to provide inhibition treatment against this type of corrosion. The prior art corrosion inhibitors for naphthenic acid environments include nitrogen-based filming corrosion inhibitors. However, these corrosion inhibitors are relatively ineffective in the high temperature environment of naphthenic acid oils.